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I'd also merge these two assertions:

match_t *m = LIST_FIRST(head);
assert(m->start == 0);
assert(m->length == 1);

into one.

When you add "benchmark timings, or some counter instrumentation", like @J_H suggested, compile with -O2/-O3.

I'd also add .DELETE_ON_ERROR to the Makefile. According to GNU make's documentation:

If .DELETE_ON_ERROR is mentioned as a target anywhere in the makefile, then make will delete the target of a rule if it has changed and its recipe exits with a nonzero exit status, just as it does when it receives a signal.

And do not comment the obvious. Though I don't see why you need the extra variable w, insert_word(root, words[i]) should suffice.

And do not comment the obvious. Though I don't see why you need the extra variable w, insert_word(root, words[i]) should suffice. And if you go to Reddit, @skeeto might see your post and teach you a thing or two about fuzz testing with AFL++ (the author of this review is not familiar with the API yet).

It also eliminates the need of having to document that the function would invoke undefined behavior if nullptrs are passed in.

It also eliminates some branches in the called function (assuming it was testing if any parameter is nullptr).

Note that this feature is not usable for functions like memcpy() because arrays of void are illegal, or for return values. A GCC extension, __attribute__((nonnull)) allows parameters to be marked as nonnull. This extension is also supported by Intel's compiler and Clang. A Clang extension allows _Nullable, _Nonnull and _Null_unspecified. This extension is not supported by GCC. A GCC extension, __attribute__((returns_nonnull)) can be used to specify that a function returns a nonnull value. This extension is also supported by Clang and Intel's compiler. I await an _Optional type in the next revision of the C Standard. There are already some proposals for it.

Note that I have changed sizeof (ac_node_t) to sizeof *node. In the case that node was changed to some other type, you would not require a similar change in the calloc() call. This eases maintainence.

And do not comment the obvious.

It also eliminates the need of having to document that the function would invoke undefined behavior if nullptrs are passed in, or if the function was testing if any parameter is nullptr, it eliminates onw or more branches.

Note that this feature is not usable for return values, local variables, or for functions like memcpy() because arrays of void are illegal. A GCC extension, __attribute__((nonnull)) allows parameters to be marked as nonnull. This extension is also supported by Intel's compiler and Clang. A Clang extension allows _Nullable, _Nonnull and _Null_unspecified. This extension is not supported by GCC. A GCC extension, __attribute__((returns_nonnull)) can be used to specify that a function returns a nonnull value. This extension is also supported by Clang and Intel's compiler. I await an _Optional type in the next revision of the C Standard. There are already some proposals for it.

Note that I have changed sizeof (ac_node_t) to sizeof *node. In the case that node was changed to some other type, you would not require a similar change in the calloc() call. This eases maintainence.

Portability:

From the Linux man page:

HISTORY         top
       <sys/queue.h> macros first appeared in 4.4BSD.
NOTES         top
       Some BSDs provide SIMPLEQ instead of STAILQ.  They are identical,
       but for historical reasons they were named differently on
       different BSDs.  STAILQ originated on FreeBSD, and SIMPLEQ
       originated on NetBSD.  For compatibility reasons, some systems
       provide both sets of macros.  glibc provides both STAILQ and
       SIMPLEQ, which are identical except for a missing SIMPLEQ
       equivalent to STAILQ_CONCAT().

1. It should be enough to define _BSD_SOURCE for sys/queue.h. Compiling with -std=gnu17 should not be necessary

2. Some compile-time detection is required to use the proper macros provided by an implementation. As the man page says, some BSDs provide SIMPLEQ instead of STAILQ.

To add to @J_H's "braces" point, see: Apple's SSL/TLS bug.

And do not comment the obvious. Though I don't see why you need the extra variable w, insert_word(root, words[i]) should suffice.